374 KICHARD EVANS. 



other hand, it has reached the condition of type C, there will 

 be chambers completely formed which have been derived from 

 the vesicular cells of the larva. In both cases the flagellated 

 cells of the larva develop into collar-cells, passing through 

 the same series of changes as they do in a larva of type D. 



In the second case, due to the fixation of a larva of type D, 

 the flagellated chambers arise almost entirely from the flagel- 

 lated layer of the larva, and scarcely at all from the cell groups, 

 which are very few in number and possibly absent. 



The divergences in the metamorphosis will be seen to be 

 diff'erences of degree which merge into one another, but which 

 nevertheless produce strikingly different appearances in the 

 early stages of development. Further, it will be evident that 

 the variations in the metamorphosis are due only to a very 

 slight degree to difl'ereuces in the age of the larvae, and depend 

 almost entirely on the divergent courses of evolution which 

 will be described in the account of the larva. 



II. Detailed Account of the Histology of the Larva and 

 OF the Metamorphosis. 



A. Histology of the Larva. 



Introductory Remarks. — In describing the histological 

 structure of the free-swimming larva and the changes through 

 which it passes before, during, and after metamorphosis, it 

 will be necessary to bring into line all the phenomena pre- 

 sented by the developing larva and the young sponge, and to 

 avoid having recourse to the easy method of explaining diffi- 

 culties as abnormalities. Further, it will be convenient to 

 avoid using the words ectoderm, mesoderm, entoderm, and 

 such terms, because they are liable to lead to confusion, and 

 to prejudge the questions of homology which may arise. 



By describing the histology of the larva, with a special view 

 to the changes which go on during the free-swimming part of 

 the life history, many of the errors of previous observers will 

 be avoided, and many of their mistakes will be explained; while 



